This invention relates generally to the rendering of latent electrostatic images visible using multiple colors of dry toner or developer and more particularly to a development apparatus including a plurality of developer housings which minimize scavenging and re-development of the first developed image by successive developer housings.
The invention can be utilized in the art of xerography or in the printing arts. In the practice of conventional xerography, it is the general procedure to form electrostatic latent images on a xerographic surface by first uniformly charging a photoreceptor. The photoreceptor comprises a charge retentive surface. The charge is selectively dissipated in accordance with a pattern of activating radiation corresponding to original images. The selective dissipation of the charge leaves a latent charge pattern on the imaging surface corresponding to the areas not exposed by radiation.
This charge pattern is made visible by developing it with toner. The toner is generally a colored powder which adheres to the charge pattern by electrostatic attraction.
The developed image is then fixed to the imaging surface or is transferred to a receiving substrate such as plain paper to which it is fixed by suitable fusing techniques.
The concept of tri-level, highlight color xerography is described in U.S. Pat. No. 4,078,929 issued in the name of Gundlach. The patent to Gundlach teaches the use of tri-level xerography as a means to achieve single-pass highlight color imaging. As disclosed therein the charge pattern is developed with toner particles of first and second colors. The toner particles of one of the colors are positively charged and the toner particles of the other color are negatively charged. In one embodiment, the toner particles are supplied by a developer which comprises a mixture of triboelectrically relatively positive and relatively negative carrier beads. The carrier beads support, respectively, the relatively negative and relatively positive toner particles. Such a developer is generally supplied to the charge pattern by cascading it across the imaging surface supporting the charge pattern. In another embodiment, the toner particles are presented to the charge pattern by a pair of magnetic brushes. Each brush supplies a toner of one color and one charge. In yet another embodiment, the development systems are biased to about the background voltage. Such biasing results in a developed image of improved color sharpness.
In highlight color xerography as taught by Gundlach, the xerographic contrast on the charge retentive surface or photoreceptor is divided into three levels, rather than two levels as is the case in conventional xerography. The photoreceptor is charged, typically to 900v. It is exposed imagewise, such that one image corresponding to charged image areas (which are subsequently developed by charged-area development, i.e. CAD) stays at the full photoreceptor potential (V.sub.cad or V.sub.ddp). The other image is exposed to discharge the photoreceptor to its residual potential, i.e. V.sub.dad or V.sub.c (typically 100v) which corresponds to discharged area images that are subsequently developed by discharged-area development (DAD) and the background areas exposed such as to reduce the photoreceptor potential to halfway between the V.sub.cad and V.sub.dad potentials, (typically 500v) and is referred to as V.sub.white or V.sub.w. The CAD developer is typically biased about 100v closer to V.sub.cad than V.sub.white (about 600v), and the DAD developer system is biased about 100v closer to V.sub.dad than V.sub.white (about 400v).
The viability of printing system concepts such as tri-level, highlight color xerography requires development systems that do not scavenge or interact with a previously toned image. Since commercial development systems such as conventional magnetic brush development and jumping single component development interact with the image receiver, a previously toned image will be scavenged by subsequent development. Great care is required to optimize the development materials and process conditions for minimum interaction. Since the present commercial development systems are highly interactive with the image bearing member, there is a need for scavengeless or non-interactive development systems, particularly one which provides the inherent attributes of magnetic brush development without image disturbance.
It is known in the art to alter the magnetic properties of the magnetic brush in the second housing in order to obviate the foregoing problem. For example, there is disclosed in U.S. Pat. No. 4,308,821 granted on Jan. 5, 1982 to Matsumoto, et al, an electrophotographic development method and apparatus using two magnetic brushes for developing two-color images which do not disturb or destroy a first developed image during a second development process. This is because a second magnetic brush contacts the surface of a latent electrostatic image bearing member more lightly than a first magnetic brush and the toner scraping force of the second magnetic brush is reduced in comparison with that of the first magnetic brush by setting the magnetic flux density on a second non-magnetic sleeve with an internally disposed magnet smaller than the magnetic flux density on a first magnetic sleeve, or by adjusting the distance between the second non-magnetic sleeve and the surface of the latent electrostatic image bearing members. Further, by employing toners with different quantity of electric charge, high quality two-color images are obtained.
U.S. Pat. No. 3,457,900 discloses the use of a single magnetic brush for feeding developer into a cavity formed by the brush and an electrostatic image bearing surface faster than it is discharged thereby creating a roll-back of developer which is effective in toning an image. The magnetic brush is adapted to feed faster than it discharges by placement of strong magnets in a feed portion of the brush and weak magnets in a discharge portion of the brush.
U.S. Pat. No. 3,900,001 disclose an electrostatographic developing apparatus utilized in connection with the development of conventional xerographic images. Developer material is applied to a developer receiving surface in conformity with an electrostatic charge pattern wherein the developer is transported from the developer supply to a development zone while maintained in a magnetic brush configuration and thereafter, transported through the development zone magnetically unconstrained but in contact with the developer receiving surface.
As disclosed in U.S. Pat. No. 4,486,089 granted on Dec. 4, 1984 to Itaya, et. al. a magnetic brush developing apparatus for a xerographic copying machine or electrostatic recording machine has a sleeve in which a plurality of magnetic pieces are arranged in alternating polarity. Each piece has a shape which produces two or more magnetic peaks. The sleeve and the magnets are rotated in opposite directions. As a result of the above, it is alleged that a soft developer body is obtained, and density unevenness or stripping of the image is avoided.
U.S. Pat. No. 4,833,504 granted on May 23, 1989 to Parker et al discloses a magnetic brush developer apparatus comprising a plurality of developer housings each including a plurality of magnetic rolls associated therewith. The magnetic rolls disposed in a second developer housing are constructed such that the radial component of the magnetic force field produces a magnetically free development zone intermediate to a charge retentive surface and the magnetic rolls. The developer is moved through the zone magnetically unconstrained and, therefore, subjects the image developed by the first developer housing to minimal disturbance. Also, the developer is transported from one magnetic roll to the next. This apparatus provides an efficient means for developing the complimentary half of a tri-level latent image while at the same time allowing the already developed first half to pass through the second housing with minimum image disturbance.
U.S. Pat. No. 4,810,604 granted to Fred W. Schmidlin on Mar. 7, 1989 discloses a printing apparatus wherein highlight color images are formed without scavenging and re-development of a first developed image. A first image is formed in accordance with conventional (i.e. total voltage range available) electrostatic image forming techniques. A successive image is formed on the copy substrate containing the first image subsequent to first image transfer, either before or after fusing, by utilization of direct electrostatic printing. Thus, the '604 patent solves the problem of developer interaction with previously recorded images by forming a second image on the copy substrate instead of on the charge retentive surface on which the first image was formed.
U.S. Pat. No. 4,478,505 issued on Oct. 23, 1984 relates to developing apparatus for improved charging of flying toner. The apparatus disclosed therein comprises a conveyor for conveying developer particles from developer supplying means to a photoconductive body positioned to define a gap therebetween. A developer supplying passage for conveying developer particles is provided between the developer supplying means and the gap. The developer supplying passage is defined by the conveyor and an electrode plate provided with a predetermined interval with the conveyor. An alternating electric field is applied to the developer supplying passage by an A. C. power source to reciprocate the developer particles between the conveyor and the electrode plate thereby sufficiently and uniformly charging the developer particles by friction. In the embodiment disclosed in FIG. 6 of the '505 patent, a grid is disposed in a space between the photosensitive layer and a donor member.
U.S. Pat. No. 4,568,955 issued on Feb. 4, 1986 to Hosoya et al discloses a recording apparatus wherein a visible image based on image information is formed on an ordinary sheet by a developer. The recording apparatus comprises a developing roller spaced at a predetermined distance from and facing the ordinary sheet and carrying the developer thereon, a recording electrode and a signal source connected thereto, for propelling the developer on the developing roller to the ordinary sheet by generating an electric field between the ordinary sheet and the developing roller according to the image information, a plurality of mutually insulated electrodes provided on the developing roller and extending therefrom in one direction, an A. C. and a D. C. source are connected to the electrodes, for generating an alternating electric field between adjacent ones of the electrodes to cause oscillations of the developer found between the adjacent electrodes along electric lines of force therebetween to thereby liberate the developer from the developing roller.
U.S. Pat. No. 4,656,427 granted to Hosaka et al on Mar. 31, 1987 discloses a method and apparatus wherein a layer of developer which is a mixture of insulative, magnetic particles and insulative toner particles is carried on the surface of a developer sleeve forming part of a magnetic brush. A latent image bearing member carrying an image to be developed is moved relative to the magnetic brush. The brush is spaced from the image bearing member and an AC field is formed across the space to effect toner transfer to the image and nonimage areas and to effect a back transfer of excessive toner.
Japanese publication 62-70881 discloses a toner separating means using a plurality of electrically biased grid wires disposed intermediate a magnet brus developer roll and an imaging surface. The two-component developer is triboelectrified and magnetic carrier is removed from the outer periphery of a sleeve by the action of the north and south poles of the magnetic poles of the magnetic brush.
U.S. patent application Ser. No. 07/171,062 filed in the name of Hays et al and assigned to the same assignee as the instant application discloses a scavengeless development system in which toner detachment from a donor and the concomitant generation of a controlled powder cloud is obtained by AC electric fields supplied by self-spaced electrode structures positioned within the development nip. The electrode structure is placed in close proximity to the toned donor within the gap between the toned donor and image receiver, self-spacing being effected via the toner on the donor. Such spacing enables the creation of relatively large electrostatic fields without risk of air breakdown.